We study hydrated model membranes, consisting of stacked bilayers of 1,2-dioleoyl-
sn-glycero-3-phosphocholine lipids, using terahertz time-domain spectroscopy and infrared spectroscopy. Terahertz ...spectroscopy enables the investigation of water dynamics, owing to its sensitivity to dielectric relaxation processes associated with water reorientation. By controlling the number of water molecules per lipid molecule in the system, we elucidate how the interplay between the model membrane and water molecules results in different water dynamics. For decreasing hydration levels, we observe the appearance of new types of water dynamics: the collective bulklike dynamics become less pronounced, whereas an increased amount of both very slowly reorienting (i.e., irrotational) and very rapidly reorienting (i.e., fast) water molecules appear. Temperature-dependent measurements reveal the interconversion between the three distinct types of water present in the system.
We studied the vibrational relaxation dynamics of the bending mode (ν(2)) of the H(2)O water molecules in the presence of different salts (LiCl, LiBr, LiI, NaI, CsI, NaClO(4), and NaBF(4)). The ...linear and nonlinear spectra of the bending mode show distinct responses of water molecules hydrating the anions. We observe that the bending mode of water molecules that are hydrogen-bonded to an anion exhibits much slower relaxation rates (T(1)~1ps) than water molecules that are hydrogen-bonded to other water molecules (T(1)=400 fs). We find that the effect of the anion on the absorption spectrum and relaxation time constant of the water bending mode is not only determined by the strength of the hydrogen-bond interaction but also by the shape of the anion.
We determine the relative positioning of water molecules in 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) membranes by measuring the rate of vibrational resonant (Förster) energy transfer between ...the water hydroxyl stretch vibrations. The rate of Förster energy transfer is strongly distance dependent and thus gives detailed information on the relative positioning of the water molecules. We determine the rate of intermolecular Förster energy by measuring the anisotropy dynamics of excited O-D stretch vibrations of HDO and D(2)O molecules with polarization-resolved femtosecond mid-infrared spectroscopy. We study the dynamics for deuterium fractions between 0.1 and 1 and for hydration levels between 2 and 12 water molecules per DOPC lipid molecule. We find that most of the water molecules hydrating the membrane are contained in nanoclusters and have an average intermolecular distance of 3.4 Å. The density of these nanoclusters increases with increasing hydration level of the DOPC membranes.
We studied the vibrational energy relaxation mechanisms of the amide I and amide II modes of N-methylacetamide (NMA) monomers dissolved in bromoform using polarization-resolved femtosecond two-color ...vibrational spectroscopy. The results show that the excited amide I vibration transfers its excitation energy to the amide II vibration with a time constant of 8.3 ± 1 ps. In addition to this energy exchange process, we observe that the excited amide I and amide II vibrations both relax to a final thermal state. For the amide I mode this latter process dominates the vibrational relaxation of this mode. We find that the vibrational relaxation of the amide I mode depends on frequency which can be well explained from the presence of two subbands with different vibrational lifetimes (~1.1 ps on the low frequency side and ~2.7 ps on the high frequency side) in the amide I absorption spectrum.
We studied the pathways of vibrational energy relaxation of the amide I (~1660 cm⁻¹) and amide II (~1560 cm⁻¹) vibrational modes of N-methylacetamide (NMA) in CCl₄ solution using two-color ...femtosecond vibrational spectroscopy. We measured the transient spectral dynamics upon excitation of each of these amide modes. The results show that there is no energy transfer between the amide I (AI) and amide II (AII) modes. Instead we find that the vibrational energy is transferred on a picosecond time scale to a common combination tone of lower-frequency modes. By use of polarization-resolved femtosecond pump-probe measurements we also study the reorientation dynamics of the NMA molecules and the relative angle between the transition dipole moments of the AI and AII vibrations. The spectral dynamics at later times after the excitation (>40 ps) reveal the presence of a dissociation process of the NMA aggregates, trimers, and higher order structures into dimers and monomers. By measuring the dissociation kinetics at different temperatures, we determined the activation energy of this dissociation E(a) = 35 ± 3 kJ mol⁻¹.
This communication study explores the relationship between self-concept and life satisfaction of single women and examines, through the use of interviews, how these two factors are connected. This ...study aims to gain perspective of why women remain single, what pressures single women face, and how a woman's life satisfaction and self-concept are affected by being single. Results showed that women remain single for a variety of reasons and while women feel pressure from family, friends, and media about their singlehood, the majority of the pressure is internal and indicated that women need a romantic relationship that will eventually lead to marriage to be satisfied with their life. It was found that the lower the self-concept a woman has, the more dissatisfied she will be if she remains single, and the higher the self-concept a woman has, the more satisfied she will be if she remains single.
Photoexcitation of graphene leads to an interesting sequence of phenomena, some of which can be exploited in optoelectronic devices based on graphene. In particular, the efficient and ultrafast ...generation of an electron distribution with an elevated electron temperature and the concomitant generation of a photo-thermoelectric voltage at symmetry-breaking interfaces is of interest for photosensing and light harvesting. Here, we experimentally study the generated photocurrent at the graphene-metal interface, focusing on the time-resolved photocurrent, the effects of photon energy, Fermi energy and light polarization. We show that a single framework based on photo-thermoelectric photocurrent generation explains all experimental results.
Graphene is a promising material for ultrafast and broadband photodetection. Earlier studies have addressed the general operation of graphene-based photothermoelectric devices and the switching ...speed, which is limited by the charge carrier cooling time, on the order of picoseconds. However, the generation of the photovoltage could occur at a much faster timescale, as it is associated with the carrier heating time. Here, we measure the photovoltage generation time and find it to be faster than 50 fs. As a proof-of-principle application of this ultrafast photodetector, we use graphene to directly
measure, electrically, the pulse duration of a sub-50 fs laser pulse. The observation that carrier heating is ultrafast
suggests that energy from absorbed photons can be efficiently transferred to carrier heat. To study this, we examine the spectral response and find a constant spectral responsivity of between 500 and 1,500 nm. This is consistent with efficient electron heating. These results are promising for ultrafast femtosecond and broadband photodetector applications.
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